New evidence about the effect of vitamin E on prostate cancer risk may make some men think twice before they pop a daily supplement. Researchers at the National Cancer Institute found that men who took a high daily dose of vitamin E had a 17 percent greater risk of developing prostate cancer.

The report, published today in the Journal of the American Medical Association, adds to a growing list of studies suggesting that supplemental vitamins have few benefits and could even be harmful.

"Many people are taking supplements because they view it as a health insurance," said Dr. Lori Minasian, one of the study's authors. "This would be an indication that it's probably not just neutral, but there is some level of harm."

Data from previous studies suggested that taking vitamin E might help protect men against prostate cancer, so Minasian and her colleagues at the NCI decided to take a closer look at the connection between the two. Beginning in 2001, they started the Select trial, studying more than 35,000 men age 55 and older in the U.S., Canada and Puerto Rico. They divided the men into four groups, each with different kinds of daily diet supplements: vitamin E, selenium, both vitamin E and selenium, or a placebo pill.

"We expected that the numbers of prostate cancer would be smaller in men taking vitamin E, but we found larger numbers instead," Minasian said.

After seven years, the researchers detected more prostate cancer in men taking one or both of the supplements, but the men taking vitamin E showed the most significant increase in their rates of prostate cancer.

And the numbers of prostate cancer diagnoses kept climbing after the men stopped taking the vitamins. Of the 8,737 men taking vitamin E, 620 of them got prostate cancer, compared to 529 of the 8,896 men taking a placebo – a 17-percent increased risk for men taking the supplement.

The authors of the study noted the findings were particularly concerning, considering how many people take supplemental vitamin E every day. Fifth percent of people over age 60 reported taking daily supplements containing vitamin E, and 23 percent of them took supplements with more than 13 times the recommended amount of vitamin E.

"We're very careful about how we take medications. We need to be equally careful about how we take supplements," Minasian said.

Some experts say that the findings from the Select trial are interesting, but the results don't necessarily mean there is an association between vitamin E and prostate cancer. Two previous studies looked at a large number of men taking the supplement and each reached different conclusions on how vitamin E affects prostate cancer.

In 2003, data from the Alpha-Tocopherol, Beta –Carotene Cancer Prevention ) trial showed that smokers taking 50 mg of vitamin E each day had a 35 percent reduction in prostate cancer. In the Physicians Health Study II, participants took the same amount of vitamin E as the men in the Select trial – 400 IU, but it had no effect on their risk of prostate cancer.

Dr. Tim Byers, the associate director for cancer prevention and control at the University of Colorado Cancer Center, said scientists should weigh the findings of all of these studies when thinking about the link between prostate cancer and vitamin E.

"If the Select trial findings were all we had, my concern might be a little higher," Byers said. "But I view this finding as contributing to an overall pattern showing little or no association [between vitamin E and prostate cancer] rather than to an overall conclusion of proven harm."

For now, experts say men shouldn't take vitamin E with the hope that it will prevent prostate cancer, and many question whether loading up on extra vitamin E is a good idea at all.

"Except for cases of clear deficiency, supplementation of vitamin E has not been shown to benefit any aspect of health, so there is no rationale for taking it," said Jack Cuzick, head of the Centre for Epidemiology, Mathematics and Statistics at the Wolfson Institute of Preventive Medicine in London.

For people who do want to take vitamin E, Minasian said getting the proper dose is the key. Men in the Select trial took a high dose of 400 IU; the Food and Nutrition Board at the Institute of Medicine recommends a daily intake of just 33 IU of vitamin E. Minasian said consumers buying supplements should check the labels and be sure the vitamin dose isn't excessive.

And it is possible to get vitamin E in foods like certain oils, meats, eggs, and leafy vegetables.

"Most people in this country, if they have a well-balanced diet, get the needed amount of vitamin E," Minasian said. "It goes back to what your grandmother said -- moderation in everything."